A fuel cell converts a fuel such as hydrogen, and an oxidant, typically oxygen or air, via electrochemical reactions into electricity, reaction products and excess heat. As shown in FIG. 1, a single fuel cell 1 is typically constituted of an electrolyte layer e.g. a proton-conducting polymer membrane 2, sandwiched between two typically flat porous electrodes 3 and 4, individually referred to as the anode and the cathode. The electrodes 3 and 4 are comprised of reactant-permeable electron-conducting substrates 3′ and 4′ with the surfaces adjacent to the electrolyte covered with thin porous active layers 3″ and 4″ containing the electrocatalysts typically comprising metals from the platinum group.
Oxidation of hydrogen at the anode catalyst layer 3″ generates protons and electrons. The protons are transferred across the electrolyte to the cathode. The electrons travel via an external circuit to the cathode. At the cathode catalyst layer 4″, oxygen is reduced by consumption of two electrons per atom to form oxide anions which react with the protons that have crossed the electrolyte layer to form water.
A plurality of single cells is usually assembled in a stack to increase the voltage and power output. Within the stack, adjacent single cells are electrically connected by means of bipolar plates (BPP) 5 and 6 positioned between the surfaces of the electrodes opposite to those covered with the catalyst layer. The BPP must be impermeable for the reactants to prevent reactant permeation to the opposite electrode. With respect to this function, the BPP is often referred to as separator, too. Flow channels 5′ and 6′ on the surfaces of the BPP provide access for the fuel to the adjacent anode 3 and for the oxidant to the adjacent cathode 4 and removal of the reaction products and the unreacted remnants of fuel and oxidant.
The electrodes each comprise a thin catalyst layer 3″ and 4″ backed by an electron-conducting porous inert substrate 3′ and 4′. The latter is often referred to as gas diffusion layer (GDL) or more generally as electrode substrate (ES). Such electrode substrates have to provide both an efficient entry passage for the fuel or the oxidant, respectively, to the catalyst layer as well as an exit for the reaction products away from the catalyst layer into the flow channel of the adjacent BPP. Within the porous electrode substrate the reactants have to be effectively transported and evenly distributed. To facilitate these mass transfer processes it is preferable that the pore fraction of the electrode substrate is large. On the other hand, the requirements of low Ohmic (electrical) resistance and adequate mechanical strength have to be fulfilled, too. Hence high porosity of an ES has to be balanced against improved through-plane conductivity.
To achieve high-volume production, it is desirable that the electrode substrate can be processed as a continuous roll material. This allows the application of cost-effective industrial scale processes for the catalyst layer deposition onto the substrate and other subsequent manufacturing steps. Moreover, a continuous roll ES provides higher homogeneity and product uniformity in comparison with ES produced in a batch-mode.
Besides the requirements discussed above the electrode material must be inert with respect to the fuels, oxidants and reaction products and stable against corrosion.
ES materials suitable for fuel cell application include carbon fibers (as non-woven or as woven cloth), metal fibers (mesh or gauze), and polymers (gauze filled with conductive particles, e.g. carbon particles). Non-wovens can be manufactured by wet laying or dry laying (e.g. paper-making) techniques.
A commonly used type of carbon fiber ES comprises carbon fibers randomly dispersed within a two-dimensional plane and mutually bond by means of a carbonized binder e.g. a carbonized resin (cf. U.S. Pat. No. 4,851,304). This electrode substrate is obtained by preparing a carbon fiber mat via paper-making technique, impregnation of the dried fiber mat with a carbonizable binder to obtain a prepreg, hot pressing of the prepreg and subsequent carbonization or graphitization of the carbonizable binder. Carbonizable binder means a binder that can be converted to elemental carbon in a high yield when heated above the decomposition temperature under an inert atmosphere. The electrode substrate made this way is an all carbon product. The carbonized binder particles contribute not only to the mechanical stability of the ES but also bring about a lowering of the resistivity in the through-plane direction which otherwise would be poor because of the mainly planar alignment of the carbon fibers within the mat.
Rather high filling levels (mass fraction of residues from the resin after carbonization is from about 45 to about 50%) are necessary to achieve sufficient through-plane conductivity. Such highly-impregnated substrates suffer from two major disadvantages: low porosity (less than 80%) which is detrimental to reactant transport, and poor flexibility. Therefore, further processing of the electrode substrate has to be carried out batch-wise resulting in high process costs and large inter-lot variations due to the difficult control of the batch-wise processing steps.
A new kind of electrode substrate has been described (EP-A 1 139471), the flexibility of which should be sufficient to allow reel to reel processing as a roll material. This electrode substrate comprises a sheet formed of carbon fibers and optionally, an admixture of expanded graphite. An organic binder which might comprise polyvinyl alcohol or a thermosetting resin like a phenolic resin or a water repellent polymer like a fluoro resin is adhered to the sheet which is subsequently heat-treated. During this optional final heat treatment, the impregnated substrates are heated to a temperature not exceeding 700° C., preferably 550° C. or lower. This temperature range is sufficient for sintering of fluoro resins like PTFE and thermosetting of phenolic or epoxy resins, but not for carbonization of a resin. According to this document, carbonization of the resin must be avoided because it would make the electrode substrate fragile. Thus, the electrode substrate obtained by this process is not an all-carbon product, and the presence of non-conductive constituents between the fibers results in a rather high through-plane resistance of 50 mOhm·cm2 of the sheets made without admixture of expanded graphite.
Another process for the production of continuous electrode substrates based on paper making technology is described in EP-A 0 791 974. Fibers selected from the group of carbon, glass, polymer, metal, and ceramic fibers, are dispersed in water with at least one catalyst component and a polymeric substance (e.g. PTFE) to obtain a dilute slurry and thereafter forming a continuous structure by the controlled deposition of said slurry onto a moving mesh bed, dewatering of the solids and drying, firing or hot pressing of the fiber containing layer under a suitable time/pressure/temperature regime, e.g. firing at a temperature in excess of 350° C. The polymeric binder used in this invention is not carbonizable. Therefore the carbon content of this kind of electrode substrate will be substantially lower than the carbon content of the above described carbon paper electrode substrates where a carbonizable binder was used. Thus the advantage of continuous processability is gained only at the expense of a reduced conductivity since the material described in EP-A 0 791 974 contains a certain fraction of non-conductive polymeric binders.
An electrode substrate which is an all carbon product and rollable (50 cm roll diameter) was recently described (WO-A 01 56 103). The carbon fiber non-woven obtained by a paper making process using a suitable binder (e.g. polyvinyl alcohol), subsequent impregnation with a carbonizable resin optionally containing conductive fillers like carbon black, hot pressing and carbonization. But to ensure rollability, it is essential that at least 40% of the carbon fibers are very fine fibers (diameter of from 3 to 5 μm). Such fine carbon fibers are not a common high-volume industrial product, but a specialty. Thus an electrode substrate made of such fibers probably will not match price targets for fuel cell commercialization.
Consequently, to enhance fuel cell commercialization electrode substrates are required which combine optimal reactant permeability and sufficient through-plane conductivity with low material costs and improved industrial processability.